1. Field of the Invention
The present invention is directed to linear radio frequency (RF) amplifiers.
2. Description of the Prior Art
Linear high-power amplifiers for amplitude-modulated RF signals are required in a number of telecommunication applications. Satellite telecommunication systems have typically utilized traveling wave tube (TWT) amplifiers. It is desirable, however, to replace such TWT amplifiers with solid state transistor amplifiers in that transistor amplifiers are typically lighter, less bulky, less expensive, and more reliable.
Attempts to replace TWT amplifiers with transistor amplifiers have historically failed; the efficiency of class A transistor amplifiers is unacceptably low and high efficiency class B or class AB transistor amplifiers have typically been unable to maintain a linear response over a spectrum of modulation frequencies which is comparable to the bandwidth of TWT amplifiers. Copending application Ser. No. 591,954 by the present inventor discloses a linear high-power transistor amplifier wherein linearity of response is maintained over a broad band of modulation frequencies by use of active biasing circuits. Experimental measurements have shown such broad-band linear transistor amplifiers to be in the order of three times as efficient as TWT amplifiers in environments requiring high carrier-to-intermodulation signal amplitude ratios. However, classes B or AB transistor amplifiers have typically been further limited in application because of the occurrence of output signal amplitude and phase distortion when high amplitude input signals are applied to the amplifier. Amplitude and phase distortion are respectively defined as the change of the amplifier gain and change of the relative phase between input and output signals in response to variations in the amplitude of the input signal. Such amplitude and phase distortions are caused, in transistor amplifiers, respectively, by saturation of the transistors, and the varactor-like inter-electrode capacitance of RF transistors.